Add sync_folders action support

Vagrant's docker provider calls sync_folders action to sync folders with the host machine and expose them as a volume in docker containers (https://github.com/mitchellh/vagrant/blob/master/plugins/providers/docker/action/host_machine_sync_folders.rb). Usually VM providers do not implement sync_folders action and docker provider uses Vagrant::Action::Builtin::SyncedFolders as a fallback. But this implementation does not work with NFS synced folders because they require additional environment configuration.
There is a workaround (https://github.com/YungSang/vagrant-docker-helper). But i would prefer to do not duplicate code but extend the host VM provider.
Add sync_folders action support

Check env for presence of nfs folder in sync_folder_config.synced_folders. If there  is no configured nfs folders for the machine nor in this hash, we will skip extra work to populate nfs related fields in the environment.
Travis: don't install custom Bundler
gemspec: Make Bundler restriction more liberal

Fixes builds on Travis CI
Merge pull request #142 from ilucker/fix/sync_folders

Add sync_folders action support

Related to mitchellh/vagrant#4011
driver: Use '--no-header' option to exclude 'INFO' from prlctl output.

Some read_* methods became universal for PD 8,9,10. We don't need to override them anymore.
driver: added 'read_vm_option' method
driver refactoring
Fixed unit tests
driver: Handle errors during 'register' and 'unregister'
action/import: Refactoring
action/import: Use template name detected by parsing 'config.pvs'
action/export: Refactoring
'prl_fs' mount on linux: added sanity check for mount options [GH-135]
Provider is parallel now

Multiple actions may be run in parallel.
Fixed Travis builds

Removed unused bsdtar
Set bundler version
Set provider priority higher than 'virtualbox'
Moved platform check to #usable? method
Version bumped to '1.3.0'
Added Parallels copyright